Grey-cartridge DS games are region-free, so any DS (DSi included) can play them. DSi-enhanced games (black-cartridge) are region-locked, along with DSiWare titles, so those cannot be played by any DS(i), only by those from the same region. If your case specifically regarding 999, because it's not DSi-enhanced (iirc), it should work fine in a UK DS. I know that there was a reissue of 999 late last year, rebranding it in line with it's sequel, Zero Escape: Virtue's Last Reward; I'm not sure if that version of 999 was released in the UK/Europe (I know the original run was exclusive to NA), but that's irrelevant since it's not DSi-enhanced anyway.